The invention generally relates to a method fluidized bed process for melting and brushing away excess solder and solder spikes from a printed circuit board.
Electronic circuit assemblies are often constructed by joining a number of integrated circuits and discrete components such as resistors, capacitors, inductors and the like to a printed circuit board having a plurality of conductive metal traces thereon. The printed circuit board is usually first fabricated by etching away portions of a metal or foil layer to define the conductive metal traces. After etching holes are drilled in the board to accept leads from the components to be connected thereto. Typically the components are placed on the board manually or by automatic insertion equipment. Following placement of the components, the printed circuit board is loaded into a conventional wave soldering machine wherein a pulse or wave of melted solder is brought into contact with the bottom side of the board opposite the side having components on it, so that solder is drawn up the leads and into contact with the conductive foil traces at the points where the leads enter the board in order to effect good electrical connections between the leads and associated conductive traces.
In some instances, however, the wave soldering machine also leaves solder spikes behind on the printed circuit board and excess solder on the conductive traces. The solder spikes and excess solder can cause inadvertent conductive bridges to be formed between various circuit elements which can cause the circuit performance to deteriorate or lead to outright circuit failure. One method previously employed for removing solder spikes from printed circuit boards has been to desolder manually portions of the board either using heated copper braid to draw the solder off by capillary action or vacuum equipment. These prior methods, however, are time consuming, often taking fifteen minutes for a single board, and expensive.
Another method for removing unwanted solder from a printed circuit board consists of immersing an entire printed circuit board in heated oil and spinning it rapidly to throw melted solder off the board. Clearly this is not a desirable process to be carried out if delicate components have already been attached to the board which would be damaged by immersion in the heated oil. Furthermore, complex machinery is required to spin the board while it is submerged in the heated oil.
U.S. Pat. No. 3,553,824 to Kozelnicky for PROCESS FOR ELIMINATING ICICLE-LIKE FORMATIONS ON SOLDERED CIRCUIT SUBSTRATES is directed to a process whereby, after wave soldering, a printed circuit board having icicle or spike-like formations of solder extending therefrom is inverted so that the solder spikes are extending upward. The fluxed soldered surface is then exposed to an infrared or radiant heat source providing sufficient heat energy to melt the solder spikes. The force of gravity pulls the softened or melted spikes down.
Another prior art approach to removing solder spikes from circuit boards is disclosed in U.S. Pat. No. 3,604,609 to Walls for APPARATUS FOR ELIMINATING ICICLE-LIKE FORMATIONS ON WAVE-SOLDERED CONNECTIONS ON CIRCUIT SUBSTRATES. That system is also directed to solving the problem of solder spike formation which accompanies the use of wave soldering machinery used to solder printed circuit boards. Walls teaches the use of a taut fine wire oriented parallel to a solder wave and immediately following it so that the wire, which is warmed above room temperature, scrapes off all of the solder spikes extending more than a pre-selected distance below the surface of the printed circuit board being soldered.
Both of these prior art processes suffer from several drawbacks. Kozelnicky does not remove any of the excess solder from the board, but rather causes it to slump down onto the surface of the board where it may spread out and form additional bridges. In addition, if solder bridges have been formed between adjacent contacts, the slumping action produced by the Kozelnicky process will not necessarily break the solder bridges. Therefore, the Kozelnicky process may leave unintentional shorts on the board. Excess solder which lies on the conductive foil traces can also form bridges. It, likewise, will not be removed by the Kozelnicky process since the solder will merely spread on the conductive traces.
The Walls process suffers from similar defects in that only a portion of a solder spike will be removed, that portion extending below the level of the scraping wire. Bridges between successive solder spikes will not be removed if the bridges are located closer to the board than the wire. It is clear that there must be some clearance between the scraping wire and the board to avoid having the scraping wire catch on the ends of leads extending below the board. Excess solder lying on the conductive foil traces on the bottom of the board will not be removed by the Walls process since the scraping wire cannot be brought into contact with it.
Therefore, the prior art suffers from the defects that the prior methods of cleaning printed circuit boards do not effectively remove all solder bridges or excess solder lying on the printed circuit board traces.
What is needed is a method for quickly and conveniently removing solder spikes and excess solder from a surface of a printed circuit board opposite the surface upon which the components are mounted. The removal process should be compatible with the board fabrication processes in that it should not cause damage to the components on the board.